Vascular stents are deployed at a narrowed site in a blood vessel of a patient for widening the vessel lumen and circumferentially supporting the vessel wall. Vascular stents desirably have a small cross-sectional diameter and/or profile for introducing the stent into the affected vessel lumen.
One type of a vascular stent is made with a piece of wire that is bent into a number of turns. Although suitable for its intended use, a problem with these bent-wire stents is that stress points are formed at each wire bend or turn. As a result, the wire stent is structurally compromised at a number of points. Furthermore, bent wire stents lack longitudinal stability. For example, a wire stent is typically positioned in a blood vessel over an inflatable balloon. The balloon expands first at opposite ends, where the balloon is not in contact with the wire stent. As a result, the wire stent is longitudinally shortened between the inflated balloon ends. With continued inflation, the middle of the balloon expands, thereby unevenly expanding the wire bends of the longitudinally shortened wire stent.
Another type of a vascular stent is made with a wire mesh that is rolled into a generally tubular shape. A problem with this stent is that the overlapping wires forming the mesh increase the stent profile, thereby reducing the effective lumen of the blood vessel. The growth of endothelial tissue layers over the wire mesh further reduces the effective blood vessel lumen. Another problem with this approach is that ion migration also occurs at the wire-to-wire contact points.
Yet another type of a vascular stent is made with a flat metal sheet with a number of openings formed in rows therein. The flat metal sheet stent also includes rows of fingers or projections positioned on one edge of the stent along the axis thereof. When expanded, a row of the fingers or projections is positioned through a row of openings on the opposite edge of the stent for locking the expanded configuration of the stent. A problem with the use of the flat metal sheet stent is that the overlapping edges of the stent increase the stent profile. Again, the stent profile and endothelial growth reduce the effective blood vessel lumen. Another problem with the use of the flat metal sheet stent is that the fingers or projections along one edge of the stent make metal to metal contact with the opposite edge of the stent. As a result, the metal edges of the stent rub during movement caused by blood flow, pulsation, and muscle movement. Yet another problem with the use of the flat metal sheet stent is that the fingers or projections extend radially outward and into the vessel wall. As a result, the intimal layer of the vessel wall can be scraped, punctured, or otherwise injured. Injury and trauma to the intimal layer of the vessel wall result in hyperplasia and cell proliferation, which in turn effect stenosis or further narrowing of the vessel at the stent site.
Still yet another type of a vascular stent is made with a piece of metal cannula with a number of openings formed in the circumference thereof. A problem with the use of a metal cannula stent is that the stent is rigid and inflexible. As a result, the stent is difficult, if not impossible, to introduce through the tortuous vessels of the vascular system for deployment at a narrowed site. Furthermore, the stent is too rigid to conform with the curvature of a blood vessel when deployed at an occlusion site. Another problem with the use of a metal cannula stent is that the stent longitudinally shrinks during radial expansion. As a result, the position of the metal cannula stent shifts, and the stent supports a shorter portion of the blood vessel wall than required.
Previous attempts to overcome flexibility problems associated with cannula stent designs have included the addition of a flexible or articulation region between the relatively rigid segments. In comparison, these flexible regions or articulations provide little radial strength. There have been clinical concerns regarding the tendency of some cannula stent designs to plastically deform at the articulations during lateral bending rather than elastically returning to the original shape. Another concern is non-uniform radial expansion of the stent during balloon inflation. A commonly observed problem with such designs is that the flexible segments do not deform outwardly in the same manner and to the same degree as the segments of higher radial strength. As a result, the stent material of the interconnection regions extends or "hangs" into the lumen of the stent (as defined by the more rigid sections). Particularly in a vascular stent, local blood flow turbulence can occur at these points that can contribute to thrombus formation.
Still another phenomenon that is especially a problem in expandable cannula type stents is the tendency of thin bars or struts to twist during expansion. Even minor manufacturing defects can create weakened bending points that contribute to this problem. A design that increases longitudinal and radial strength and stability, has fewer articulations, and evenly distributes bending stresses is less prone to twisting and non-uniform expansion. Distribution of bending stresses is also an important factor in determining a stent's susceptibility to fatigue. Articulations designed to provide flexibility between tubular non-flexible sections are typically subject to stresses during deformation that can lead to breakage. The likelihood of breakage can increase when the articulation points are welded rather than being part of the cannula wall.
For coronary applications, the ideal stent would be thin-walled, of unitary construction to eliminate welds, and have high radial strength with good endoluminal coverage to prevent restenosis. In addition, the stent would have a low profile on the balloon to reach small vessels, yet would have a good expansion ratio with low recoil following delivery to prevent migration or becoming undersized for the diameter of the lesion. An ideal coronary stent would be able to follow tortuous vessels during introduction while maintaining its shape without plastically deforming. Another desirable property is the ability of the stent to remain crimped upon the balloon so that slippage does not occur and, as a result, eliminates the need for endcaps or another means to hold the stent on the balloon. Although high radial strength is needed, the ideal coronary stent must be able to be elastically flexible over millions of bending cycles to accommodate changes in the vessel due to systole and diastole. The ideal stent should deploy uniformly at the target site without twisting, migrating, or taking on an accordion or scalloped appearance, should retain its original axial length during deployment, and should be visible under radiographic imaging as an aid in placement. While most available stents can adequately meet a limited number of these objectives, design compromises have restricted the utility and efficacy of these stents for certain clinical applications.